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Section 19: Soil Exposure Pathway - Nearby Population Threat

19.1 Overview of the Threat

  • This threat assesses the relative risk that nearby individuals (residents, students, and day care children) may travel to the site and come into contact with hazardous substances.
    • "(T)he weighting of the nearby population relative to the resident population has been reduced to better reflect the relative levels of exposure for those threats" (Preamble to the Hazard Ranking  System (HRS), 55 FR, page 51560, Section N).
  • The threat will contribute very low scores to the soil exposure pathway unless:
    • The area of observed contamination is 9 acres or more and includes an area that is regularly used for public recreation.
    • Surrounding population density is heavily urban so that many people are potentially exposed to the observed contamination.
  • Highlight 9-18 in the HRS Guidance Manual may help you estimate the nearby population score. Urban population densities in major U.S. cities averages about 5,000 people per square mile and can run as high as 20,000 in some areas like to Bronx or Queens in New York.
    Guidance Manual, Highlight 9-18 : Approximate Nearby Population Threat Scores, page 385

  • The threat to nearby individuals, as contrasted to resident individuals, can usually be addressed effectively and much more quickly by the removal authorities of CERCLA rather than by recourse to placement on the National Priorities List (NPL).

19.2 Areas of Observed Contamination

  • The area of observed contamination is determined in the same manner as explained in Section 17 of this training.
  • From this area of observed contamination used for the resident population threat, two sub-areas are deleted for the nearby population threat.
    • Areas with an attractiveness/accessibility value of 0 in HRS Table 5-6, namely "Physically inaccessible to the public, with no evidence of public recreation use." The HRS Guidance Manual gives the example of "Area off-limits to unauthorized personnel at guarded and fenced military base or industrial complexes." This exclusion is parallel to the exclusion of sources with a containment value of 0 in the migration pathways.
    • Residential properties. These properties are excluded from evaluation for attractiveness/accessibility and, therefore, cannot have a value greater than 0.

  • These sub-areas are not evaluated for likelihood of exposure, waste characteristics, or travel distance for targets.

Soil Population Threat

19.3 Likelihood of Exposure

  • The likelihood of exposure is estimated from two considerations:
  • The larger the area of observed contamination for the nearby population threat, the higher the value.
  • The more attractive and accessible the contaminated areas in terms of recreational use, the higher the value.
    • Highlight 9-21 in the HRS Guidance Manual on page 391 gives examples for attractiveness/accessibility values. If more than one value can be assigned to the overall area of observed contamination, assign the highest.
      Guidance Manual, Highlight 9-21, page 391

    • Note the emphasis on recreational activities. These activities were judged to be "the most likely to result in exposures to contaminated surficial materials" (Preamble to the HRS, 55 FR, Section N, page 51560).
    • The values for area and attractiveness/accessibility are combined in HRS Table 5-8, page 51648.
      HRS rule, Table 5-8, page 51648

19.4 Waste Characteristics

The values for toxicity and hazardous waste quantity are assigned in the same manner as for the resident population threat (Section 18-4 of this training), except:

  • Delete from consideration any area of observed contamination that cannot be assigned a value for attractiveness/accessibility greater than 0.
  • Because the toxicity may be based on a more limited set of substances and the quantity value on a smaller area, the value for waste characteristics may be lower than for the resident population threat.

19.5 Travel Distance for Nearby Targets

  • Travel distance in categories of less than 0 to 1/4 mile, less than 1/4 mile to 1/2 mile, and less than 1/2 mile to 1 mile must be mapped so that targets can be properly distance-weighted. This mapping may vary from the distance rings for the ground water or air pathways in two ways:
    • Distance is measured from areas of observed contamination with an attractiveness/accessibility factor value greater than 0 rather than from HRS sources with containment values greater than 0.
    • If there is a natural barrier to travel such as a river, distance is measured via the nearest crossing rather than straight line between the source and the target.

Guidance Manual, Highlight 9-22, page 393: Establishing Target Distance Categories for Nearby Threat: Method 1

19.6 Nearby Targets

  • Only two factors are evaluated: nearby individual and population within 1 mile. These two target types are treated and evaluated for potential exposure rather than actual exposure.
  • Population within 1 mile: Examine HRS Table 5-10, 55 FR, page 51649 carefully. Compare the value assigned for 1,200 people within 1/4 mile travel distance (41) with the value assigned in HRS Table 3-12, page 51604 for 1,200 people who receive drinking water from a well within 1/4 mile (1,633).

The value for the nearby targets in the soil exposure pathway receive only 1/40 the value of potential targets in the drinking water threats (ground water or surface water). This difference results from the exposure scenarios that underlie the different exposure routes.

19.7 Questions and Answers

What areas of observed contamination should be included for the nearby population threat?

Do not count the three areas on residential properties and exclude the area that is fenced and guarded. You are left with two areas of observed contamination: the area around the pile and the area on the school property.

The area of observed contamination is about 200' by 500'. Part of the area lies within a fenced but abandoned industrial property and part extends onto the neighboring Little League ballpark. What is the value for likelihood of exposure?

125.    The attractiveness/accessibility values are 10 and 100 unless you documented public recreation within the fenced area. Select the higher. The area value for 100,000 square feet is 20.

Nearby Individual:  Look at HRS Table 5-9, page 51649. What is the maximum value that can be assigned?

1.     As you were already warned, the values assigned to nearby populations "have been reduced to better reflect the relative levels of exposure." Note in the footnote that if a value has been assigned for resident individual, none is assigned to nearby individual. This is parallel to the practice in the migration pathways of assigning only the highest value (Level I, Level II, or potential) for the MEI risk.

What is the estimated value for population within 1 mile at a rural site with a population density of 100 people per square mile? Assume that the areas within the three distance rings are 0.2, 0.6, and 2.6 square miles respectively.

The population value is 0.21.

  100 x 0.2 = 20:  value = 0.4

  100 x 0.6 = 60: value = 0.7

  100 x 2.6 = 260: value = 1.0

sum = 2.1 x 1/10 for potential = 0.21

EXERCISE:     Lets complete the nearby population threat with some quick what-if calculations based on the following site information. The site is a small, bankrupt wood preserver with observed contamination documented in the process area and drip yard for a total of about 50,000 square feet. The maximum toxicity value for wood-treating substances found in the sampling is 10,000. The site is not secured from access. There are no resident targets.

Likelihood of exposure:     The site can probably be considered to be moderately accessible. Lets assume a worst case that we will be able to document some evidence of public recreation (= 50). This combined with an area value of 20 gives a likelihood of exposure value of 25 out of a possible 500 points (Table 5-8). The true area of observed contamination is probably larger than what has been sampled but it would have to be 2.5 times larger (greater than 125,000 square feet) to raise the value for likelihood of exposure to 50. After you estimate the pathway score, decide whether this sampling effort is worthwhile.

Waste Characteristics:     The value for hazardous waste quantity from Table 5-2 is less than 1 (50,000/34,000) but the minimum value for the soil exposure pathway is 10 (text from page 51592, bottom half of the left column). Enter 10 x 10,000 into Table 2-7 and find a value of 18 for waste characteristics.

Targets:     Since there are no resident targets, we'll assume that someone lives within 1/4 mile and assign a tentative value of 1 for nearby individual and add this to the value of 0.21 for population that was calculated in response to the previous question.

Nearby threat score:     25 x 18 x 1.21 = 544.5. Note, in the pathway formula in Table 5-1, page 51646, that you do not divide by 82,500 until after you have summed the score for the resident and the nearby threats.

Since the resident threat is zero, the pathway score is 0.007.

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